Many pharmaceutical compositions include adjuvants in order to increase activity, antigenic potency and to enhance stability of the formulation. In this regard, vaccine compositions often include immunological adjuvants to enhance cell-mediated and humoral immune responses. For example, depot adjuvants are frequently used which adsorb) and/or precipitate administered antigens and which serve to retain the antigen at the injection site. Typical depot adjuvants include aluminum compounds and water-in-oil emulsions. However, depot adjuvants, although increasing antigenicity, often provoke severe persistent local reactions, such as granulomas, abscesses and scarring, when injected subcutaneously or intramuscularly. Other adjuvants, such as lipopolysacharrides and muramyl dipeptides, can elicit pyrogenic responses upon injection and/or Reiter's symptoms (influenza-like symptoms, generalized joint discomfort and sometimes anterior uveitis, arthritis and urethritis).
Despite the presence of such adjuvants, conventional vaccines often fail to provide adequate protection against the targeted pathogen. In this regard, there is growing evidence that vaccination against intracellular pathogens, such as a number of viruses, should target both the cellular and humoral arms of the immune system.
More particularly, cytotoxic T-lymphocytes (CTLs) play an important role in cell-mediated immune defense against intracellular pathogens such as viruses and tumor-specific antigens produced by malignant cells. CTLs mediate cytotoxicity of virally infected cells by recognizing viral determinants in conjunction with class I MHC molecules displayed by the infected cells. Cytoplasmic expression of proteins is a prerequisite for class I MHC processing and presentation of antigenic peptides to CTLs. However, immunization with killed or attenuated viruses often fails to produce the CTLs necessary to curb intracellular infection. Furthermore, conventional vaccination techniques against viruses displaying marked genetic heterogeneity and/or rapid mutation rates that facilitate selection of immune escape variants, such as HIV or influenza, are problematic. Accordingly, alternative techniques for vaccination have been developed.
Particulate carriers with adsorbed or entrapped antigens have been used in an attempt to elicit adequate immune responses. Such carriers present multiple copies of a selected antigen to the immune system and promote trapping and retention of antigens in local lymph nodes. The particles can be phagocytosed by macrophages and can enhance antigen presentation through cytokine release. Examples of particulate carriers include those derived from polymethyl methacrylate polymers, as well as microparticles derived from poly(lactides) and poly(lactide-co-glycolides), known as PLG. Polymethyl methacrylate polymers are nondegradable while PLG particles biodegrade by random nonenzymatic hydrolysis of ester bonds to lactic and glycolic acids which are excreted along normal metabolic pathways.
Recent studies have shown that PLG microparticles with entrapped antigens are able to elicit cell-mediated immunity. For example, microencapsulated human immunodeficiency virus (HIV) gp120 has been shown to induce HIV-specific CD4+ and CD8+ T-cell responses in mice (Moore et al., Vaccine (1995) 13:1741-1749). Additionally, both antibody and T-cell responses have been induced in mice vaccinated with a PLG-entrapped Mycobacterium tuberculosis antigen (Vordermeier et al., Vaccine (1995) 13:1576-1582).
While offering significant advantages over other more toxic systems, antigen-entrapped PLG microparticles suffer from some drawbacks. For example, the production of microparticles is difficult and involves the use of harsh chemicals that can denature the antigen and destroy the immunogenicity thereof. Furthermore, antigen instability can occur due to the high shear forces used to prepare small microparticles and due to interfacial effects within the emulsions used.
The use of antigens adsorbed to microparticles avoids these drawbacks. However, reports on the immunogenicity of microparticles with adsorbed antigen have been mixed. In fact, experimenters have postulated that antigens must be entrapped in microparticles in order to achieve an adequate adjuvant effect. See, e.g., Eldridge et al., Infect. Immun. (1991) 59:2978-2986; Eldridge et al., Seminars in Hematology (1993) 30:16-25; Nakaoka et al., J. Controlled Release (1995) 37:215-224; Sah et al., J. Controlled Release (1995) 35:137-144; and Duncan et al., “Poly(lactide-co-glycolide Microencapsulation of Vaccines for Mucosal Immunization” in Mucosal Vaccines (Academic Press, Inc., 1996).
More particularly, microparticle-encapsulated and -adsorbed ovalbumin have been shown to prime cellular immune responses in vivo and induce mucosal IgA responses when administered orally. However, entrapped antigen elicited better responses than adsorbed antigen (O'Hagan et al., Vaccine (1993) 11:149-154). Coombes et al., Vaccine (1996) 14:1429-1438 also describes experiments using both ovalbumin-encapsulated and -adsorbed microparticles. Antibody responses to the adsorbed antigen were significantly lower than those elicited by administration of entrapped ovalbumin. Finally, antigen-specific CTL responses have been reported in mice using a short synthetic peptide from the circumsporozoite protein of Plasmodium berghei microencapsulated in biodegradable microspheres or adsorbed on empty microspheres (Men et al., Vaccine (1997) 15:1405-1312).
However, none of the above studies describe the use of antigen-adsorbed microparticles, using viral antigens, to stimulate cell-mediated immune responses. Accordingly, there is a continued need for effective and safe adjuvants for use in a variety of pharmaceutical compositions and vaccines.